Very large, well-formed crystals of an alum can be prepared by suspending a small crystal by a thread in a saturated solution of the alum, as shown in Fig. 84. The small crystal slowly grows and assumes a very perfect form. [Illustration: Fig. 84] ~Other salts of aluminium.~ While aluminium hydroxide forms fairly stable salts with strong acids, it is such a weak base that its salts with weak acids are readily hydrolyzed. Thus, when an aluminium salt and a soluble carbonate are brought together in solution we should expect to have aluminium carbonate precipitated according to the equation 3Na_{2}CO_{3} + 2AlCl_{3} = Al_{2}(CO_{3})_{3} + 6NaCl. But if it is formed at all, it instantly begins to hydrolyze, the products of the hydrolysis being aluminium hydroxide and carbonic acid, Al_{2}(CO_{3})_{3} + 6H_{2}O = 2Al(OH)_{3} + 3H_{2}CO_{3}. Similarly a soluble sulphide, instead of precipitating aluminium sulphide (Al_{2}S_{3}), precipitates aluminium hydroxide; for hydrogen sulphide is such a weak acid that the aluminium sulphide at first formed hydrolyzes at once, forming aluminium hydroxide and hydrogen sulphide: 3Na_{2}S + 2AlCl_{3} + 6H_{2}O = 2Al(OH)_{3} + 6NaCl + 3H_{2}S. ~Alum baking powders.~ It is because of the hydrolysis of aluminium carbonate that alum is used as a constituent of some baking powders. The alum baking powders consist of a mixture of alum and sodium hydrogen carbonate. When water is added the two compounds react together, forming aluminium carbonate, which hydrolyzes into aluminium hydroxide and carbonic acid. The carbon dioxide from the latter escapes through the dough and in so doing raises it into a porous condition, which is the end sought in the use of a baking powder. ~Aluminium silicates.~ One of the most common constituents of rocks is feldspar (KAlSi_{3}O_{8}), a mixed salt of potassium and aluminium with the polysilicic acid (H_{4}Si_{3}O_{8}). Under the influence of moisture, carbon dioxide, and changes of temperature this substance is constantly being broken down into soluble potassium compounds and hydrated aluminium silicate. This compound has the formula Al_{2}Si_{2}O_{7}.2H_{2}O. In relatively pure condition it is called kaolin; in the impure state, mixed with sand and other substances, it forms common clay. Mica is another very abundant mineral, having varying composition, but being essentially of the formula KAlSiO_{4}. Serpentine, talc, asbestos, and meerschaum are important complex silicates of aluminium and magnesium, and granite is a mechanical mixture of quartz, feldspar, and mica.